Just got a beater 92 XJ. I push in the gas and once 25MPH hits, the car wobbles and feel like a earth quake . when i let go off the gas pedal..it goes away..once i push it back in a bit ..the wobble is back.

Don't overlook, tire inflation/balancing, and the obvious looseness of any parts. Look for shiny metal as evidence something is moving around.

First I'd check tightness of Track Bar, and its bracket bolts. This always been my Death Wobble issue.

As a result, I am in process of installing new Trackbar drop Bracket and bolts. Added a Trackbar Bracket Brace (HIGHLY recommended based on my experience)! I also installed a Steering Box Brace. I am getting ready to replace all steering components with Currie HD Steering system._________________

i'd agree with everything jppilot61 said, also i would check the u-joints sounds alot like what mine did when they went? is it lifted? if so and drive line angle was never corrected it could be reeking havoc on your u joints.